This invention relates to the field of log processing equipment and particularly to a new type of fuel pellet manufacturing process and equipment.
In recent years a new type of residential wood pellet burning stove is becoming popular around the world. These pellet stoves provide a number of significant advantages over previous fiber fuel stoves including a low rate of particulate emissions, high energy efficiency, cleanliness and general convenience of use. Many localities in which air quality has become a serious problem in winter such as in the northwestern states of the United States have passed stringent air quality legislation which has increased the demand for pellet stoves.
In the past, the pellet fuel for these pellet stoves has been produced by grinding saw mill residues and compressing the ground material at high pressures like 20,000 psi through extrusion dies forming small cylindrical pellets having a diameter around 9/32 inch and less than 1.5 inches in length. In recent times, there is less saw mill residue available and the cost of raw material for pellets has increased. Accordingly, pellet manufacturers have begun chipping trees for their raw material and grinding the chips.
The capital investment in pellet equipment to establish a pellet mill is over $1 million 1992 U.S. dollars. Also, pellet mill operating costs add significantly to pellet costs because of the large amount of power required for grinding and heating with steam and compression.
There is another type of wood processing equipment for fuel known as a hog. This machine typically includes a number of large interleaving hooks which rotate together with tremendous torque and which clamp onto a wooden piece and crush and break it as the hooks pass by one another. After the crushing, the material is ground up and separated by screens into sizes desired. This is a low production rate operation and is generally used in conjunction with waste lumber less than 4 inch diameter. This type of processing is used to prepare fuel called "hog fuel" for commercial boiler operation.
There is still another type of wood processor known as a chipper which is sometimes used for processing fuel. In general, most chippers are used to make thin chips which are used in paper manufacture. The prior art chippers are generally divided into two classes. The first type employs a massive large diameter disk which is rotated at high torque and velocity about its axis and which has a plurality of radially mounted knives protruding from the face of the disk. Appropriately these prior art devices are known as disk chippers. In these devices the material to be chipped is presented to and pressed against the face of the disk at various angles and at various positions with respect to the center of the disk. These devices only accurately control two dimensions of the chip they produce. The width of the chip is controlled by the height that the knife protrudes above the face of the disk and the thickness of the chip is determined by the feed rate of the work piece. The third dimension of the chip is determined by the grain lines of the wood and the angle and path of the knife impact. Typically the third dimension is between 1 to 2 inches. In the paper industry, the chip length is not a critical dimension. Cutting directly across the grain by feeding the work place into the disk perpendicularly to the face of the disk is avoided because of the fragmentation of the wood along the grain lines.
Another form of chipper machine is in current use. This one employs a large diameter hollow cylinder and is called a Lily Pad chipper, a name which it has acquired because its primary use has been in chipping log ends which are called Lily Pads. In this chipper, a plurality of short knives are mounted around the periphery of the cylinder. The work place which is shorter than the diameter of the cylinder gets jammed down between one side of the cylinder and a side wall where it is chipped away by the approximately 2 inch knives which have their leading edge parallel to the axis of the cylinder. This chipper also has no control over the length of the chip. The other two dimensions are fixed by the height of the knife above the cylinder and the length of the knife.
Both of these prior art chippers are designed for handling large diameter fir logs and they are accordingly large and expensive devices. The disk chipper is capable of a higher production rate which derives from the fact that several knives across the entire log diameter make a full cut for each revolution. In the lily pad chipper only one knife hits the log at any axial position for each revolution of that chipper.
The disk chipper has been made in both a portable and stationary variety. The portable device can go directly into the forest and permits some reduction in the handling and transportation expenses. The costs of logging and chipping for a team of 3 loggers and 2 equipment drivers is about $600 per hour in 1991 dollars. This implies the smallest diameter trees which are economical to chip knowing the feed rate of the apparatus for any given diameter. In 1991, the economic break even point for a logging operation in Oregon was approximately an 8 inch diameter tree. This has resulted in cessation of harvest activities for large forest areas in which a high percentage of the ground coverage is less than 8 inches in diameter. It is a fact that there is as much total fiber content in a forest which has not been thinned or otherwise cultivated as in a cultivated forest having stands of large trees. Accordingly, there is a very large acreage of species of hardwoods, such as madrone and white oak in the world which are not lumber or paper chip resources but could be resources for pellet fuel if production costs were reduced to permit their economic utilization.
In view of the above, there is a long felt need for a less expensive means to utilize available hardwood which is now being ignored because it is too expensive to produce. Also, there is a growing worldwide recognition that dwindling petroleum resources need to be conserved and that renewable resources should be more fully employed as long as air quality standards can be maintained. The residential pellet stove is a good answer to these problems and the market for wood fuel pellets has been growing faster than the supply. The design of pellet stoves vary, but most systems employ a gravity feed pellet bin and an auger fuel feeder. In order to flow properly in the feed bin, the pellet must be smooth enough on its exterior to slide or it will hang up. Also, the pellet cannot be shaped like a match stick or it will bridge over the auger entrance, or jam the auger by getting stuck in the auger side clearance.
It is an object of our invention to provide machinery requiring less capital investment and which is less expensive to operate to manufacture fuel pellets and to enable use of fiber resources which are abundantly available but which heretofore have had little commercial use.
It is another object to reduce pellet manufacturing costs by providing apparatus to directly manufacture such pellets from a log without the requirement to first reduce the log to chips which then need to be finely ground and extruded at high temperature and pressure.
It is still a further objective to provide a machine which is able to process both hardwood and softwood small diameter logs directly into pellets without the shattering and fragmenting which accompanies chipping and with minimal residual unavailable fiber and with essentially no waste or kerf.
It is a still further objective to provide a machine design which is simple and which can be made portable for in-forest operations.
It is a still further object of our invention to provide a solid wood pellet having a smooth exterior which will flow easily in a gravity feed pellet bin and which will not bridge over or jam in the pellet stove auger.